1. Field of the Invention
This invention relates to a laser which contains a substantially planar waveguide. More particularly, it relates to such a laser wherein the waveguide along the direction of laser light propagation is comprised of varying combinations of high-gain and low-gain regions.
2. Description of the Prior Art
A laser is a device which has the ability to produce monochromatic, coherent light through the stimulated emission of photons from atoms or molecules of an active medium which have been excited from a ground state to a higher energy level by an input of energy. Such a device contains an optical cavity or resonator which is defined by highly reflecting surfaces which form a closed round trip path for light, and the active medium is contained within the optical cavity.
If a population inversion is created by excitation of the active medium, the spontaneous emission of a photon from an excited atom, molecule or ion returning to its ground state can stimulate the emission of photons of identical energy from other excited atoms, molecules or ions. As a consequence, the initial photon creates a cascade of photons between the reflecting surfaces of the optical cavity which are of identical energy and exactly in phase. A portion of this cascade of photons is then discharged through one or more of the reflecting surfaces of the optical cavity.
Excitation of the active medium of a laser can be accomplished by a variety of methods. However, the most common methods are optical pumping, use of an electrical discharge, and the passage of an electric current through the p-n junction of a semiconductor laser.
Semiconductor lasers contain a p-n junction which forms a diode, and this junction functions as the active medium of the laser. Such devices, which are also referred to as laser diodes, are typically constructed from materials such as gallium arsenide and aluminum gallium arsenide alloys. The efficiency of such lasers in converting electrical power to output radiation is relatively high and, for example, can be in excess of 40 percent.
Laser diodes ordinarily contain a waveguide which serves to confine the laser light to a specific region within the optical cavity. This waveguide can be either a gain-guided structure or an index-guided structure. The gain-guided structure is formed temporarily during injection of pumping current into the laser diode and is believed to result, at least in part, from formation of a thermally induced refractive index gradient within the active layer and cladding layers during operation of the device. On the other hand, index-guided structures contain a built-in refractive index profile which is parallel to the p-n junction and serves to confine the light in a region parallel to the junction.
A stripe geometry is conventional for the waveguide of a laser diode where the width of the waveguide is comparable to the wavelength of the laser light. With such a geometry, lowest order or fundamental lateral mode operation can be achieved. However, it has been recognized that an increased amount of output radiation can be obtained through the use of a broad and substantially planar or so-called slab waveguide. Such a slab waveguide has a width which is typically tens of times larger than that of the stripe waveguide and, accordingly, can contain a larger volume of active material and afford a higher output power at reduced current and power densities.
Unfortunately, the use of a slab waveguide carries with it an undesirable loss of lateral mode control. Since the width of the slab waveguide is large enough to support a multitude of lateral modes, the output beam from the laser has a profile along the width of the waveguide which is very irregular, and this profile can change with both time and pumping level. The output beam profile from such a slab waveguide is also a function of any minute defects and inhomogeneities. These defects and inhomogeneities result in filamentation, a phenomenon wherein portions of the active region act quasi-independently of each other to afford a near field pattern which consists of bright filaments in a limited area rather than a smooth intensity distribution across the width of the waveguide. At the present time, there is no completely satisfactory method to cure these problems without seriously complicating the laser diode fabrication procedure, which increases the device cost.
Various types of multiple-stripe lasers have been developed in an effort to overcome the problems which are associated with slab lasers. For example, the optical coupling of adjacent, parallel, stripe lasers has been described by J. E. Ripper et al., Appl. Phys. Lett., 17, 371 (1970). U.S. Pat. No. 4,255,717 (Scifres) discloses a laser device which contains a plurality of adjacent stripe lasers wherein a portion of the radiation established in any one stripe is deflected and coupled into one or more of the adjacent stripes. Further, an asymmetric, offset stripe laser diode array has been described by D. F. Welch et at., Electronics Lett., 21, 603 (1985). However, all of these approaches involve the coupling of single stripe lasers and do not solve the above-described problems which are associated with a slab laser.
J. Salzman et al., Appl. Phys. Lett., 49, 611 (1986) have described a diode laser having a slab waveguide which possesses identical periodic perturbations as a consequence of multiple ridges extending some 10 .mu.m in a direction normal to the facets. This laser is multifilamentary in character and produces a single lobed and nearly diffraction limited far-field pattern. However, this reference fails to suggest a laser diode having a slab waveguide which is perturbed in a manner which is not periodic in an identically repetitive manner. In addition, J. Salzman et al., Appl. Phys. Lett., 46, 218 (1985), have described the fabrication and operation of a GaAs diode laser with both mirrors having a convex shape and wherein the lateral width of the gain region was defined by an 80 .mu.m contact stripe. It is reported that this laser operated stably in a single lateral mode over a range of injection currents. However, the optical cavity of this laser did not contain a combination of high-gain and low-gain regions.